Commercial desulfurization techniques only remove a portion of the sulfur present in coal or in coal chars. This is inadequate for purposes of complying with air purity standards which limit the amount of sulfur dioxide produced by the use of coals, chars, and other carbonaceous materials as a fuel. The need for a relatively sulfur-free char has therefore become very important, especially in view of dwindling supplies of oil and natural gas and abundant supplies of coal.
One method of desulfurizing coal is by first crushing and grinding the coal and introducing the coal fines into a fluid medium to allow the sulfur in the form of FeS.sub.2 (pyrite) to form a separate layer which is mechanically removed from the coal. This technique, however, does not remove the FeS.sub.2 embedded in the uncrushed coal and does not remove any of the sulfur bound to organic compounds.
Another method is to treat coal and char with hydrogen gas which leaches out the sulfur by reacting therewith to form hydrogen sulfide gas. This method may work with coal but not with char because when coal is converted into char, chemical reactions occur between the sulfur and the inorganic and organic compounds in the coal which "fixes" the sulfur in the char and renders the sulfur more resistant to leaching operations.
Hot, molten caustic has also been used to leach sulfur from finely divided coal. This approach, however, removes only a partial amount of the sulfur and would not appear to be suitable for desulfurizing char due to the "fixing" of the sulfur when coal is converted to char. Furthermore, sulfur bound with organic compounds is leached out in the molten caustic method only after the coal is in a semi-fluid or plastic condition. Char does not become semi-fluid or plastic and would therefore not appear to be usable in a molten caustic system.